Are you absolutely certain it's hayfever? Reason I ask is that a few years ago I had what I thought was extremely bad hayfever, same as your description, and I took anti-histamines for a week with no effect. Someone suggested it might actually be a cold so i took cold pills and felt better within a few hours.
I have also had the absolutely horrendous tree pollen hayfever too, but found it was only really horrible for a few days.